Lightning arrester and method of making the same



June 17, 1930. J. R. McFARLlN 1,763,667l

LIGHTNING ARRESTER AND METHOD OF MAKING THE SAME June 17,` 1930. 1. R. McFARLlN 1,763,667

LIGHTNING ARRESTER AND METHOD, OF MAKING THE SAME Filed April 5, 1926 2 Sheets-Sheet 2 0.59 D rsu, wo 15o j M @GEMM um o L50 r'oo 2.50 boo Patented June 17, 1930` UNITED STATES PATENT oEFlcE JOHN ROBERT HOFARLIN, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR T0 ELEC- TRIO SERVICE SUPPLIES COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A COB- PORATION OF PENNSYLVANIA y LIGHTNING 'Aaansran AND METHOD or MAKING THE sana Application led April 5,

My invention provides an improved apparatus permitting the discharge to ground of abnormal electric charges or currents, such as lightning or other high voltage disturbances due to static conditions, resonance, etc., but interrupting the fiow to ground of power or dynamic currents from electric conductors or instruments following such discharges and preventing such How under normal conditions.

I have discovered that masses of discrete particles of infusible refractory materials of limited conductivity or comparatively low y specific resistance, such, for instance, as silicon carbide, possess under certain conditions, peculiar attributes of conductance and thermal capacity not otherwise possessed by them and that by incorporating such materials, preferably in granular crystalline form, under the proper conditions in the ground discharge path of a lightning arrester, particularly of the air gap type, such properties may be utilized to greatly enhance the effective- `ness, durability, stability and simplicity of the arrester.

I have ascertained that the conductance characteristics for high voltage discharges of arresters comprising such materials are dependent primarily upon the relationship of the length to the cross sectional area of the mass incorporated in the discharge path and the size ofthe particles or granules comprised in the mass, and that the dynamic failure point of such arresters is primarily dependent upon the length'of the discharge path through the particles and the size of such particles, and secondarily upon the freedom of the mass from compound or imperfect crystals lor impurities.

The applied low frequency current voltage necessary to break down or arc-over an' air gap of given clearance in a discharge path containing such crystals of a given size increases relatively slowly and in a comparatively uniform manner with increases in the ratio between the length and cross sectional area of the crystalliferous mass, hence the low frequency arc-over voltage value of an arrester embodying such elements is dependent primarily upon the length of the air gap or 1926. Serial No. 99,809.

gaps rather than upon the relative dimensions of the mass of crystals or the size of the crystals. But when voltages and currents of high frequency or steep wave front are applied to such an arrester having an air gap or gaps of given clearance, the impedance of the arrester varies markedly at certain points with variations in the ratio between the length and cross sectional area of a mass of crystals of a. given size, and differences in the slze of the crystals cause marked variations in the ratios at which the marked changes in impedance occur, viz:

The impedance ofthe arrester rises rapidly as the ratio between the length and cross sectional area of the mass of crystals is increased until a certain well defined ratio is reached, such ratio being dependent upon the size of the granules comprised in the mass. From this point, there is a sharp and svbstantially constant decreaseto a Well defined minimum in the impedance of the arrester as the ratio between the length and cross sectional area of the mass of crystals is increased. until a further well defined ratio is reached. which ratio is dependent upon the size of the granules comprised in the mass.

Further increases -in the ratio between the length and cross sectional area of the mass of crystals result in substantially constant and rather steep increases in the impedance of the arrester. the steepness of the increase becoming greater as the size of the granules comprised in the mass decreases.

The ratio of length to cross sectional area of a mass of crystals to secure minimum impedance to high voltage discharges decreases as'the size of the granules comprised in the mass decreases, and substantially the same minimum impedance to high voltage discharges may be secured in masses of the same cross section composed of granules of any size by properly proportioning the length of the mass relatively to its cross sectional area. Also the ratio of length to cross sectional area of a mass of crystals of given size to secure minimum impedance tou high voltage discharges decreases as the cross sectional area of the mass increases, and the minimum impedance obtainable with a mass of crystals of given siz'e becomes higher as the diameter of the mass increases. The critical ratios for crystalliferous masses of different cross sectional conformations and comprising particular crystals are readily determinable by test.

The dynamic failure point of such an arrester, or the capacity of its crystal mass to suppress an arc and prevent power current from following a disruptive discharge across and air gap, is dependent primarily upon the length of the discharge path through the crystalliferous mass and the size of the crystalline granules comprised therein. With the increase of the length of the discharge path through a crystalline mass composed of crystals of given size, the dynamic failure voltage is increased, or, conversely, with the decrease in the size of granules forming a discharge path of given length the dynamic failure voltage of the arrester is increased.

The functional properties of such granular masses are -not only aected by the proportional dimensions thereof and by the size of the constituent grains, but the mass should be as free as possible from impurities, weak crystals or conglomerate grains made up of more than one crystal in order that the arrester may have high dynamic failure effectiveness.

By the utilization of the discovery of the foregoing phenomena, I am enabled to produce alightning arrester for any given service having the desired characteristics that The break down or arc over value on normal voltage is only slightly greater than the normal voltage of the circuit on which the arrester is used in order that the arrester may be sensitive to rises of voltage on the circuit of normal frequency as well as of high frequency or steep wave front:

The impedance to high voltage discharges may be made substantially a minimum:

The dynamic failure point may be made sufficiently higher than the normal voltage of the circuit to preclude the possibility of destruction of the arrester during operation.

A lightning arrester made in accordance with and embodying my invention comprises electrodes affording a spark gap proportioned relatively to the normal current voltage and primarily controlling the arcing over thereof: and a mass composed of particles, preferably crystals, sized, and forming a discharge path of a length, proportioned to the desired dynamic failure value or arc suppressing capacity. The mass may have such cross sectional area relatively to its length that its impedance to high frequency discharges will be a minimum, that is to say, will be less than the maximum impedance interposed by a mass of similar cross section and composition and shorter length and less than the impedance of substantially similar value to said maximum interposed by a mass of similar cross section and composition and greater -length. It isd objectionable to incorporate in an arrester a mass in which low impedance to high frequency discharges is secured by making the-ratio of length to cross section smaller than that resultin in the first maximum impedance because of the resulting low dynamic failure value of the arrester.

Lightning arresters made in accordance with m improvements are highly efficient in protecting electrical apparatus from various classes of lightning and high voltage disturbances since the air gap or gaps in the discharge path may be adjusted to break down or arc over at voltages but 'slightly higher than normal line voltage and no fixed resistance in the usual sense is employed. rlhe thermal capacity of the device is such as to permit discharges to a ground over considerable periods of time without sintering or fusin the crystalline conductive material, therey avoiding internal short circuiting and minimizing the danger of self-destruction. The operating characteristics of the arrester are not appreciably changed by repeated discharges, and the initial high etiiciency of the device is maintained over long periods of time with little or no necessity for inspection or expense for maintenance. Devices embodying myA improvements are durable, simple and convenient in construction and may be manufactured in large quantities at low cost.

The characteristic features and advantages of my improvements will further appear from the following description and the accompanying drawings, in which are illustrated an arrester constructed in accordance with and embodying my invention and data determinative of the form and construction thereof.

In the drawing, Fig. l is a longitudinal sectional view of a lightning arrester embodying my improvements; Fig. 2 is an'end view thereof; Fig. 3 is a diagrammatic layout of the discharge path of an arrester; Fig. 4 shows typical curves covering several of the properties of a lightning arrester assembly, the circuit of which is shown in Fig. 3; Fig. 5 shows a group of graphic curves illustrating the relations existing between arrester impedance to discharges of high frequency or steep wave front and the size of the crystalline granules and length of path therethrough, the diameter of the mass remaining constant; Fig. 6 shows a similar group of curves but with the diameter of the mass greater than the diameter of the mass producing the conditions shown in Fig. 5; and Fig. 7 shows a graphic group of curves illustrating the relations existing between the length of discharge path and size of granules and the dynamic failure Voltage of a mass of crystals.

As illustrated in Figs. 1 and 2 of the drawings, a non-conducting cylindrical casing 1,

mesma? preferably made of porcelain, has seated on the corrugated bottom 2 thereof an apertured conducting plate 3 connected with the ground conductor 4 which projects through the bottom thimble 5. The plate 3 may be positioned by a non-conducting rod 6 which passes through the aperture therein and through a counterbored hole 7 in the casing bottom; the rod having on the end thereof a lock washer 8 vand nut 9 by which a conducting disk 10l is drawn against the washer 11 to clamp in place the plate 3 and against the non-conducting packing 12, such as felt, to securely seal the lower end of the arrester while maintaining a good electrical path between the disk 10 and the ground.

A mass of silicon carbide crystals 13 is packed in the casing in electrical contact with the disk 10. Grains of uniform size and which will pass through screens Yof from twelve meshes per inch to forty meshes perinch have been found very satisfactory in discharging lightniner to ground and in interrupting the iow of usual dynamic currents after a lightning discharge. The particular size of grains used is, however, dependent u on the permissible impedance to disc arges, the desired or allowable dynamic failure oint, the relative dimensions of the mass o crystals, and to some extent upon the number and length of air gaps in the discharge path, since it lhas been found that marked increases in the gap length somewhat increases the sharpness of the curve by which impedance and dynamic failure may be graphicall illust-rated.. The grains should be as near y perfectly crushed as possible as the resence of twenty per cent or ymore o wea crystals or conglomerate grains has been found to lower the dynamic failure limit of the device below that which is commercially desirable, and a smaller percentage of weak or conglomerate grains deleterious effect. For instance, by substituting a mass of crystals containing approximately eighty-eight per cent of', perfectly crushed grains for a mass of crystals containing ap roximately eight -two per cent of perfect fy crushed grains t e dynamic failure tIoint o an arrester was raised substantially fty per cent, or from approximately 4,000

volts to approximately 6,000 volts.

The granular mass is confined by conducting disks 14 and 15 loosely sleeved on the rod 6; the disk 15 having a flange 16 spun over to press the gasket 17, of suitable packing material, firmly against the inner wall of the casing.

Dished spark gap electrodes 18, each pair suitably spaced from one another by the respective nesting insulators 19, are sleeved on the rod 6 withthe lowermost electrode in contact with the disk 15 and the uppermost electrode electrically connected through the conducting washer 20 with the anged conducthas a substantiallyv the cap 21 is slightly spaced in the assembled arrester from the end of the casing 1, so as to avoid any compressive stresses on the container 1 yb tightening the nuts 22 and 9; If desired, tie rod G may be omitted and the apertures therefor eliminated.'

A clamping strap or band 23 encircles the flange of the cap 2l and is tightened by a bolt and nut 24; one end of the band being looped to form a terminal 25 to which a line wire 26 may be soldered or otherwise fastened.

A porcelain lid or cover 27 is telescoped over the end of the casing 1 with its flange overlying the band 23 and terminal 25, and the channel between the casing and cover is filled with suitable sealing material to exclude moisture and dirt and retain the cover in place.

he casing 1 may be fixed to a suitable base by means of a bracket 28 seated between the beads 1 on the easing, the bracket legs being clamped together by a bolt and nut 29 and bent to form attaching feet 30.

In operation, a lightning or other high voltage discharge entering the arrester through the conductor 26 flows through the clamping band 23, cap 2l, washer 20, electrodes 18, and across the air gaps between the electrodes, through the washers 15, 14,- crystalline conducting material` 13, conductors 10, 11, 3, and 4 to ground. The length and diameter of the f crystalline conducting mass and the size of-its constituent grains, having been properly proportioned for lthe desired functions, the arrester not only facilitates the discharge of the high voltage disturbance, and thereby etlicientlv protects the electrical apparatus with which it is connected, but also properly interrupts the flow of dynamic current following the lightning discharge to ground by suppressing the arcs across the air gaps not later than the end of the rst half cycle of the dynamic current voltage wave; such air gaps normally affording an impedance greater than the break down or are over value of the dynamic current.

Fig. 3 illustrates a typical circuit of an arrester such as that illustrated in Figs. 1 and 2, and Fig. 4 shows typical curves graphically illustrating characteristics of such arrcsters.

In this graph 4), the curve A illustrates the slight variations in the break down Cil representing voltage break down values, while the abscissae represent axial lengths of the mass of crystals in terms of fractions or multiples of its diameter.

Curve B illustrates the variations in the impedance of an arrester, under voltages and currents of high frequency or steep ywave front, with the same air gap and the same crystal masses as in curve A; the scale for the absciss and the methods of determining the co-relation between ordinates and abscissae being the same as for curve A, but the ordinates of the curve being based on the scale at the left of the figure. These values of impedance are expressed in terms of voltage drop across the discharge path as measured by a calibrated sphere gap in shunt thereto. lV ith a given value of discharge current, high impedance in the discharge path is manifested by a high voltage drop across the discharge path; lower impedance by a lower voltage drop across the discharge path.

Curve C of Fig. 4 shows, to an ordinate scale similar to the ordinate scale for curve B, the dynamic failure of the arrester assembly. This curve of dynamic failure voltage values is determinable by connecting the arrester assembly to a power circuit of large capacity and from which, through suitable transforming means, various voltages may be impressed'across the terminals of the arrester. A moment-ary voltage in excess of that required to break down or arc over the air gan of the arrester is superimposed on the circuit and destroys the dielectric properties of the arrester gap, thereby permitting a iow of dynamic or power current from the circuit of How through the arrester. Successive tests of this character may be made at gradually increasing values of circuit voltage until the arrester fails to cut olf or interrupt the current flow therethrough.

As shown graphically by the curves of Fig. 7, the dynamic failure voltage of an arrester comprising a crystalline mass of given length relative to diameter increases as the size of the granules decrease, and the .dynamic failure voltage of an arrester comprising a crystalline mass composed of granules of given size increases as the length relative to diameter of the discharge path through the crystals increases. For example, a lightning arrester utilizing 16 mesh silicon carbide crystals with a two inch length of path will fail on dynamic voltages of approximately 2000, while an arrester utilizing 100 mesh silicon carbide crystals with a two inch length of path will not fail until the dynamic voltage reaches approximately 8500 volts.

From the groups of graphic curves shown in Figs. 5 and 6, it will be seen that for a given diameter of mass, the ratio of length to diameter of the mass decreases with the size of the granules to secure the minimum impedance to high frequency dicharges, but

for any given diameter the minimum impedance to high frequency discharges attainable with various sizes of granules does not greatly vary. The group of curves shown in Fig. 6 represents results secured with masses of crystals larger in diameter than the masses used to secure the results plotted in Fig. 5, and from a comparison of the groups it will be seen that the ratio of length to diameter of the mass to secure minimum impedance to high frequency discharges decreases as the diameter of the mass increases, and the impedance of a mass of given ratio composed of granules of given size increases somewhat as the diameter of the mass increases.

The steepness or slope of the various curves for different sizes of granules increases as the size of the granules decreases, thus bringing closer together the first maxima of any curve with an impedance of similar value on the curve at a point where ratio of length to mass diameter is greater. In view of the increasing steepness of these impedance curves for granules of the finer sizes and the greater exactness required to build lightning arresters whose impedance is a minimum for the particular elements employed, I do not desire to limit myself to lightning arresters constructed according to my invention whose impedance is the literal minimum for the particular elements employed, but intend to comprehend within such terms as used in the claimsstructures or operations wherein the impedance will be near to the minimum value and between the first maxima of any impedance curve and an impedance of substantially similar value on the curve at a point where the ratio of length to mass diameter is greater.

From the foregoing it will be understood that to produce a lightning arrester having the desirable characteristics hereinbefore enumerated, there is established an air gap distance between electrodes which, in cornbination with any particular size of granular crystalline conducting material of desired length of path, will give :i predetermined value of low frequency break downor arc over voltage and a dynamic failure voltage of a predetermined higher value than normal line voltage. There may be then determined the proper relative diameter or cross sectional area of the crystalline mass necessary to impart to the arrester a substantially minimum impedance to discharges of high frequency or steep wave front.

It is apparent that if the mass of silicon carbide crystals is not properly proportioned, the impedance of the arrester to high frequency discharges will increase whether the ratio of length to diameter is greater or less than the proper ratio, whereas if the mass is properly proportioned the impedance to high frequency discharges will lie at the point X of the curve B of Fig. l, or at least at some oint on the curve B between the first maxima impedance point Y and a point Z where the impedance 1s substantially the same as at Y. The point of failure to suppress an are maintained by dynamic current will lie at the point m on the curve C sufficiently higher than the normal voltage of the system to preclude the possibility of the arrester being destroyed in operation; and the break down or arc over value on normal voltage will lie at the point fn, on curve A and be but slightly more than normal circuit voltage and primarily dependent on the air gap distance between'the arrester electrodes.

While in the particular examples given, by way of illustration, I have herein specified the dimensions of the mass in terms of length and diameter, it is apparent that the same may be expressed in terms of length and sectional area, hence I do not intend to limit myself to crystalline masses of circular cross section since the characteristics exhibited by such masses may be readily duplicated with masses of non-circular cross section.

It is frequently preferableA for practical reasons that the discrete mass of crystals be of circular cross section and of a size that will not pass though a screen having forty meshes per inch, and crystals anywhere between this size and those that will not ass through a screen having less than tweve meshes per inch are found most satisfactory for dise tribution s stems employing relatively' low voltages. he size of the mass is proportioned to the size of the crystals and conse- '.quently to conform with existing commercial conditions as to dynamic failure in such low voltage distribution systems, the crystal mass is usually made of greater length than diameter.

Arresters havinga crystal mass in the discharge path thereof in accordance with my invention have a low specific resistance to dynamic current; an impedance to high voltage discharges lower than would be present in a similar arrester having a similar mass of crystals of shorter length and a dynamic failure point higher than the dynamic failure point of a similar arrester having a similar mass of crystals of shorter length.

In the underlying theory and construction, my invention departs widely from arresters employing resistance blocks, rods, plates or haphazard masses of crystals as spark gap electrodes or as component parts, or resistance blocks of large cross sectional area and short length. The mass of crystalline conducting material used as a component part of my arrester is of comparatively low specific resistance; and the area of a section taken longitudinally of the mass in the direction of low therethrough is usually large relativeI to the area of a section taken perpen lcularly therethrough, being of comparatively low specific resistance, the crystalline mass permits dynamic or power current followinga lightning discharge to flow therethrough, but this flow is promptly interrupted by the inherent design of the arrester and the. characteristics of the crystalline material employed.

Vhile other crystalline materials may be employed either alone or in combination, I prefer to use one simple compound, such as silicon carbide, in the formation of the crystalline body, as I find that by so doing I am readily enabled to design an arrester for either low or high voltage service, to secure an arrester the internal resistance of which does not vary as theresult of molecular re-arrangement or chemical changes, and even though the tempera-ture co-eiicient of resistance of silicon carbide is negative, there is little probability of sinterino' or fusing, with consequent destruction ofD the arrester, by the flow of power current.

From the foregoing description of my invention, it will be apparent that my improved lighting arrester is not only extremely elicient, but is very reliable and durable, and while I have described specific examples by way of illustration, it will be obvious that various changes may be made without departing from the scope and spirit of my invention.

Having described my invention, I claim:

1. A lightning arrester comprising electradesV formin a spark gap proportioned relatively to t e normal current voltage of the circuit protected by the arrester; and a discrete mass in series with the gap and composed of particles of refractor material of limited conductivity sized and formin a discharge path of a length, proportione to the desired dynamic failure of the arrester; said mass having such cross sectional area relatively to its length as interposes a substantially minimum impedance to high voltage discharges.

2. A lightning arrester having in the discharge path thereof a crystalliferous mass of low specific resistance to dynamic current and having its dimensions proportioned to afford impedance to high voltage discharges lower than the impedance of a similar mass of shorter length and having a length providing a dynamic failure point higher than the dynamic failure point of a similar mass of shorter length.

3. A lightning arrester having in the discharge path thereof a gap in series with a erystalliferous mass of granules of low specic resistance to dynamic current,said gap being proportioned relatively to the normal current voltage of the circuit protected by the arrester, the size of said granules relatively'to the length of the discharge path through the mass being proportioned to the normal otential of the line protected to provide a esired arc suppressive capacity.

4. A lightning arrester having in the discharge path thereof a cylindrical mass of silicon carbide granules, said mass having a length greater than its diameter and being composed of grains proportioned to the size 0f the mass and of a size that will not pass through a screen having more than forty meshes per inch. t

5. A lightning arrester comprising elec trodes forming a gap and a mass of crystals connected therewith, the size of said crystals being proportioned to the length of the discharge path formed by the mass to suppress an arc maintained across said gap by a pre- Adetermined potential.

6. A lightning arrester comprising elec trodes forming a spark gap and a mass of silicon carbide crystals connected therewith having the size of the crystals and the length of the conducting path formed thereby proportioned to provide an arc suppressive effect greater than the arc suppressive eect of a similar mass of minimum practicable length and the area and length of the path being proportioned to provide conductance to high voltage discharge greater than the conductance of a similar mass of minimum practicable length.

7 A lightning arrester comprising a mass of silicon carbide crystals so proportioned to the length of the path formed thereby as to provide a dynamic failure value for dynamic current higher than the dynamic failure value for such current of a shorter similar mass and the area of the conducting path being so proportioned to its length as to provide an impedance to high voltage discharges lower than the impedance thereto formed by said shorter similar mass.

8. A lightning arrester comprising a mass of crystalline material having a conductance for high voltage discharges variable with the ratio between its length and volume and arc suppressive capacity variable with its length, said mass having such geometrical constants relatively to the size of its grains that its arc suppressive capacity is higher than the capacity of a similar mass of shorter length and its conductance for high voltage discharges is greaterthan the conductance of such similar mass of shorter length.

9. A lightning arrester comprising graular crystalline material decreasing to a minima in conductance of high voltage discharges with increases in the lengtlrrelatively to the volume thereof, and increasing to a maxima in conductance of high voltage discharges with further increases in the length relatively to the volume thereof, and decreasing in conductance of high voltage discharges with further increases in the length relatively to the volume thereof, the mass of material comprised in said arrester having such geometrin cal constants relatively to the size of its grains that the ratio between its length and volume is greater than the ratio causing such minimum conductance and less than the higher ratio causing a conductance equal to the minimum conductance.

10. A lightning arrester comprising a mass of such dimensions and composed of particles of such size relatively to such dimensions that the potential across same under a given high frequency discharge is substantially a minimum as compared with other dimensioned masses of such crystals.

11. A lightning arrester comprising a mass of loose silicon carbide crystals of substantially uniform size such that the grains will pass through a screen having 12 meshes to the inch but will not pass through a screen having 40 meshes to the inch, the diameter of the mass being less than its length and proportioned to interpose substantially minimum impedance to high frequency discharges.

12. A lightning arrester comprising a casing having therein a mass of loose crystalline granules, and means for compressing said granular mass without longitudinal strain on the Walls of said container.

13. A lightning arrestor comprising a casing having a conductor passing therethrough, a plate connected with said conductor, an insulating tic rod connected with said plate, electrodes on said rod, spacers between said electrodes, said electrodes and spacers being clamped together by said rod, a plate adjacent to the opposite end of said electrodes from said plate lirst named, and a mass of loose granules of silicon carbide contacting with said plate, a plate at the end of said mass opposite the plate second named.

14. The method of controlling the dynamic failure and impedanceto high voltage discharges of a gap lightning arrester which comprises incorporating in the discharge path a granular mass of silicon carbide, proportioning the length of the mass and size of the granules to give a desired arc suppressive effect, and proportioning the cross section of the mass relatively to its length to interpose a substantial minimum impedance to high voltage discharges.

15. The method of producing a lightning arrester which comprises proportioning an electrode gap relatively to the normal current voltage of the circuit protected by the arrester to afford a desired impedance to arcing connecting in series with the gap a mass of discrete particles, proportioning the length of the mass and the size of the particles to the desired dynamic failure of the arrester, and proportioning the cross section relatively to the length of said mass as to afford a substantially minimum impedance to high voltage discharges.

16. A lightning arrester comprising a mass of silicon carbide crystals having such geometrical constants relatively to the size of its grains that its impedance to dynamic current is higher than the impedance thereto formed by a shorter similar mass and its impedance to high voltage high frequency discharges is lower than the impedance thereto formed by said shorter similar mass.

17. A lightning arrester comprising a casing having a series of gap electrodes spaced by insulators, a mass of discrete material of low conductivity in series With said electrodes and means comprising a tie rod passing through said electrodes and mass and exerting a compressive force thereon.

18. The method of forming a lightning arrester which comprises proportioning an electrode gap relatively to a desired dynamic current break down value, connecting in series with the air gap electrodes a mass of material of limited conductivity and composed of discrete particles, proportioning the length of said mass and the size of said crystals to a desired dynamic failure value, and so proportioning the cross section of said mass relatively to its length as to afford a substantially minimum impedance to discharges of high frequency and voltage.

19. A lightning arrester comprising electrodes forming a spark gap proportioned relatively to the normal current voltage of the circuit protected by the arrester; and a dis.

. crete mass in series with the gap and composed of particles of infusible refractory material of limited conductivity the size of the particles being proportioned to the length of the discharge path formed by the mass to suppress an are maintained across said gap by a predetermined potential.

20. A lightning arrester comprising electrodes for the formation of an arc, said electrodes being spaced proportionately to the normal current voltage of the circuit protected and means for suppressing an arc formed between said electrodes and comprising a discrete mass connected With one of said electrodes and'composed of particles of infusible refractory material of limited conductivity forming a discharge path, the length of the path being so proportioned to the gap between the electrodes and the size of the articles as to provide a substantially maximum are suppressive effect within permissible limits of impedance to high voltage current.

21. A li htning arrester having in the discharge pat thereof a mass of silicon carbide crystals, said mass having a length greater than the diameter of a circle equal in area to the area of a section of said mass normal to its length and beine' composed of grains proportioned to the size of the mass and of a size that will not pass through a screen having more than forty meshes per inch and providing ressive effect withinl the limits of permisslve impedance to high voltage current.

0' a substantially maximum arc supf 22. The method of making a lightning arrester which comprises proportioning an electrode gap relatively to the normal current voltage of the circuit protected by the arrester, connecting With one of said elecg suppressive effect proportioned to the noI- l,

mal current voltage of the circuit protected by the arrester.

In testimony whereof Ihave hereunto set my hand this 31st day of March, 1926.y

JOHN ROBERT MCFARLL 

